The waterproof membranes or coatings are laminated onto the back of the material. This means that they permanently prevent water from permeating the fabric. The outer fabric protects the membrane or coating against wear/strains from the outside.
The fabric exterior can become saturated with water if the fibres are not water-repellent. A saturated outer material will become heavy, cause heat loss and gives your skin the impression that the clothing is porous. Furthermore, saturated outer material is less breathable and significantly heavier. It is therefore important to make the exterior of laminated fabrics or materials featuring a waterproof coating hydrophobic, i.e. water-repellent. This is what requires the further treatment.